Guidelines were presented in a Google Webmaster Central Blog post.
Although the post dates back to 2012, it remains Google's most recent statement
on the matter and the guidelines remain relevant.

No cloaking - Cloaking is showing one set of content to your
users and a different set of content to search engine bots by specifically
identifying them and purposely feeding them different content.

Target, as a platform, has been configured to treat search engine
bots the same as any user. This means that the bots might get included in tests
you are running, if they are randomly selected, and "see" the test variations.

Use rel="canonical" - Sometimes an A/B test needs to be set
up using different URLs for the variations. In these instances, all variations
should contain a
rel="canonical" tag that references the original
(control) URL. For instance, if Adobe were testing its home page using
different URLs for each variation, the following canonical tag for the home
page would go in the
<head> tag for each of the variations:

<link rel="canonical" href="http://www.adobe.com"
/>

Use 302 (temporary) redirects - In the instances where
separate URLs are used for the variation pages in a test, Google recommends
using a 302 redirect to direct traffic into the test variations. This tells the
search engines that the redirect is temporary and will only be active as long
as the test is running.

A 302 redirect is a server-side redirect, and Target, along with
most optimization providers, uses client-side capabilities. Therefore, this is
an area where Target is not fully compliant with Google's recommendations.
This, however, impacts only a small fraction of tests. The standard approach
for running tests through Target calls for changing content within a single
URL, so no redirects are necessary. There are instances when clients need to
use multiple URLs to represent their test variations. In these instances,
Target uses the JavaScript
window.location command to direct users to test
variations, which does not explicitly signify whether the redirect is a 301 or
302.

Although we continue to look for viable solutions to completely
align with search engine guidelines, for those clients that must use separate
URLs for testing, we are confident that proper implementation of the canonical
tags mentioned above mitigates the risk associated with this approach.

Run experiments only as long as necessary - We believe "as long
as necessary" to be as long as it takes to reach statistical significance.
Target
provides best practices to determine when your test has
reached this point. We recommend that you incorporate the hardcoded
implementation of winning tests into your testing workflow and allot the
appropriate resources.

Using the Target platform to "publish" winning tests is not
recommended as a permanent solution, but as long as the winning test is
published for 100% of users 100% of the time, this approach can be used while
the process of hardcoding the winning test is completed.

It's important to consider what your test has changed as well.
Simply updating the color of buttons or other minor non-text-based items on the
page will not have any influence over your organic rankings. Changes to text
should be hardcoded, however.

It's also important to consider the accessibility of the page you're
testing. If the page is not accessible to the search engines and was never
designed to rank in organic search in the first place, such as a dedicated
landing page for an email campaign, then none of the considerations above
apply.

Googles states that following these guidelines "should result in your
tests having little or no impact on your site in search results."

In addition to these guidelines, Google also provides one more
guideline in the documentation to their Content Experiments tool:

"Your variation pages should maintain the spirit of the content on
your original pages. Those variations shouldn't change the meaning of or your
user's general perception of that original content."

Google states as an example that "if a site's original page is loaded
with keywords that don't relate to the combinations being shown to users, we
may remove that site from our index."

We feel that it would be difficult to unintentionally change the
meaning of the original content within test variations, but we do recommend
being aware of the keyword themes on a page and maintaining those themes.
Changes to page content, especially adding or deleting relevant keywords, can
result in ranking changes for the URL in organic search. We recommend that you
engage with your SEO partner as part of your testing protocol.